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NCERT Revision Notes for Chapter 6 Physical and Chemical Changes Class 7 Science | Classrankers.com

Physical and Chemical Changes

NCERT Revision Notes for Chapter 6 Physical and Chemical Changes Class 7 Science

CBSE NCERT Revision Notes

1

Topics

Answer

  • Types of changes -Physical and Chemical 
  • Difference between Physical and Chemical Change
  • Few Examples of Chemical Changes:
   -Copper Sulphate Reaction 
-Crystallization
-Rusting  
  • Protection of Rusting
Exercise

2

Types of Changes - Physical and Chemical Changes 

Answer


Physical Change

→ Properties such as shape, size, colour and state of a substance are called its physical properties.

→ A physical change in which a substance undergoes a change in its physical properties is called a  physical change.


Chemical Change

→  A change in which one or more new substances are formed is called a chemical change.

→ A chemical change in which a substance undergoes a change in its chemical properties is called a chemical change.

Exercise

3

Difference between Physical and Chemcial Changes 

Answer


Physical Change

Chemical Change

1. The chemical composition of a

substance does not change.

1. The chemical composition of a

substance changes.

2. Most changes are reversible.

2. Most changes are irreversible.

3. No new substances are formed.

3. No new substances are formed.

For example:

Ice → Water → Steam

 For example,

Paper → Ashes


Exercise

4

Examples of Chemical Changes 

Answer

• Burning of Magnesium Ribbon

→ Magnesium ribbon burns with a brilliant white and leaves behind a powdery ash when completely burnt.

Magnesium (Mg) + Oxygen (O2) → Magnesium oxide (MgO)


Dissolving magnesium ash in water

Magnesium oxide (MgO) + Water (H2O) → Magnesium hydroxide [Mg(OH)2]

*Note:- Melting of wax is Physical as well as Chemical Change. 


Copper Sulphate Chemical Reaction



→ When copper sulphate reacts with iron, the colour of the solution changes from blue to green due to the formation of iron sulphate.

• Copper Sulphate solution (blue) + Iron → Iron Sulphate solution (green) + Copper (brown deposit)


Crystallization

Crystallization is an example of physical change. The process of crystallization is used for

purification of some substances.



Rusting



→ Rusting is an example of chemical change.

Iron (Fe) + Oxygen (O2, from the air) + water (H2O) → rust (iron oxide Fe2O3)

→ Presence of both air and water is essential for rusting to take place.

→ Rusting can be prevented by cutting the contact of either air or water or both with iron.

Exercise

5

Protection from Rusting

Answer


→ The same can be done by greasing, oiling, painting, and galvanizing iron.

→ The process of depositing a layer of zinc on iron is called galvanization.

→ Rusting can also be checked by alloying iron with other elements. Stainless steel is an alloy of iron with carbon, chromium, nickel, and manganese.

Exercise